In an optical receiving device, an optical signal received by a photodiode (PD) is converted into a current signal, and further converted by a transimpedance amplifier (TIA) circuit into a voltage signal. FIG. 9 is a block diagram showing an example of the arrangement of a general TIA circuit. A TIA circuit 100 is a multistage amplifier circuit including amplifiers 22, 40, and 50. The TIA circuit 100 also includes an automatic gain control (AGC) circuit 200 that includes a gain control unit 21 and controls the gain of the amplifier 22 of the first stage. The gain control unit 21 controls a feedback resistor Rf of the amplifier 22 by a control voltage Vagc, and thus controls the gain of the amplifier 22 such that the amplitude of a signal output from the amplifier 22 maintains a predetermined value.
In optical communication, since the received light intensity changes depending on the communication distance, it is necessary to amplify any signals ranging from a weak (dark) optical signal to a strong (bright) optical signal while ensuring low noise and low distortion. For this purpose, a TIA circuit for optical communication often has a function of controlling the amplification gain in accordance with the magnitude of an input signal strength, that is, the magnitude of a received light intensity. The TIA circuit makes the gain large when the received light intensity is high, and makes the gain small when the received light intensity is low. An AGC circuit that automatically executes such gain control has been input into practical use in various architectures.
The AGC circuit controls the gain by a certain time constant. The function of determining an optimum gain and controlling it by the AGC circuit will be referred to as an “AGC function”, and the time constant used by the AGC circuit when controlling the gain will be referred to as the “time constant of the AGC function” hereinafter.
In the arrangement example of FIG. 9, the time constant of the AGC function is decided by the magnitudes of a resistor Ragc and a capacitor Cagc of the AGC circuit 200. If the time constant of the AGC function is too short, following to a logic change in the input signal occurs, and therefore, it may be impossible to obtain a desired output. This is because, for example, if the gain is made small when the input signal is at High level, and the gain is made large at Low level, the output level at High level and the output level at Low level almost equal, and the output amplitude consequently becomes small. In general, considering the baud rate or encoding method of input data, the time constant of the AGC function is designed such that it attains a sufficient length to grasp the average input amplitude.
However, in a case in which intermittent optical signals (burst signals) are received in a PON (Passive Optical Network) system or the like, if the time constant of the AGC function is too long, correct reception cannot be performed until an optimum gain is set. For this reason, an enormous preamble signal needs to be included in a transmission frame, and as a result, the communication efficiency greatly lowers.
For example, in the standard of 10G-EPON standardized by IEEE802.3av, the burst response time should be 800 ns or less in total in a TIA circuit and a limiting amplifier of the subsequent stage. The TIA circuit preferably responds within about 400 ns. In a general TIA circuit for a continuous signal, however, the time is as long as several s to several ms.
Hence, for burst communication, the time constant of the AGC function is set relatively short, or switching between different fixed gains is done on a burst basis, thereby ensuring the response speed and the dynamic range as an amplifier circuit (for example, see non-patent literature 1).
However, it is difficult to simultaneously implement smooth gain control completely proportional to an input amplitude and a quick response. If the circuit lacks one of them, it causes degradation in a bit error rate (BER) characteristic representing the relationship between input optical power and a bit error amount. For this reason, in burst communication of a data rate more than 10 Gbps, a mechanism for relieving a predetermined amount of bit errors by a forward error correction (FEC) function is introduced.
As described above, conventionally, the time constant of the AGC function of a TIA circuit is fixed. Hence, a TIA circuit for continuous optical communication that sets a relatively long time constant of the AGC function cannot respond to a burst signal. If a TIA circuit that sets a relatively short time constant of the AGC function for burst communication is used for continuous optical communication, consecutive identical digits become long because of the encoding method. Particularly, the BER characteristic for input optical power in an error free near-field region degrades. To apply the TIA circuit for burst communication to a continuous signal, expensive signal processing such as FEC is needed. This is undesirable in a network, for example, Ethernet® that needs to build a system at low cost.
As a solution to this problem, a method of connecting a capacitive element or a resistive element as an external component to the outside of an IC chip on which a TIA circuit is integrated and controlling the time constant is considerable (for example, see non-patent literature 2). In this method, however, if there exist a plurality of portions to increase or decrease capacitors or resistors in the circuit, as many terminals (pads) used to connect external elements are needed. It is also necessary to ensure a space to mount external elements in an optical module on which the TIA chip is mounted.